Preheat oven to 350°. Lightly grease a 13x9x2-inch pan with butter; set aside.Combine apples, cranberries, sugar, and lemon juice, and place in prepared pan.In a bowl, combine brown sugar, oats, and cinnamon. Using a pastry blender, cut in butter until crumbs form. Spread mixture over apples and cranberries.Bake for 45 minutes.Serve warm with ice cream, if desired.